1. Field of the Invention
The present invention relates generally to photoelectric transducers, image reading devices, image forming apparatuses, and image reading methods.
2. Description of the Related Art
In scanners, CCDs (Charge Coupled Devices) have conventionally been used as photoelectric transducers. However, recent demands for speedup have focused a spotlight on CMOS (Complementary Metal Oxide Semiconductor) linear image sensors (CMOS sensors). CMOS sensors are analogous to CCDs in converting incident light into an electrical signal using a photodiode (PD). However, CMOS sensors differ from CCDs in that CMOS sensors perform charge-voltage conversion and output resultant voltages downstream near every pixel. Furthermore, because CMOS sensors use a CMOS process, a CMOS sensor can incorporate a circuit(s), such as an ADC (Analog-Digital-Converter), therein. Furthermore, CMOS sensors can be more advantageous than CCDs in terms of speed when configured to include ADCs, each of which is provided for one or more pixels, and operate the ADCs in parallel.
Some CMOS sensors may use a modulated clock (SSC (Spectrum-Spread Clock)) in clocking. The SSC is used to reduce an influence of electromagnetic interference (EMI); however, use of the SSC in an analog circuit can disadvantageously cause stripes to appear on an image. The stripes result from that drive timing is modulated by the SSC. Even if the SSC is not used in the analog circuit, when ADC and/or another logic circuit is provided on the same chip as the CMOS sensor, noise resulting from the SSC can reach the analog circuit via the logic circuit and disadvantageously cause similar image stripes to appear.
To counter the above problem, for example, Japanese Unexamined Patent Application Publication No. 2001-268355 (Patent Document 1) discloses an image reading device configured to align phases of a modulation period of a spread-spectrum clock generator circuit with reference to a main-scanning line-sync signal using a modulation phase aligner, thereby making phases of modulated period coincide with each other, so that a main-scanning line has the same periodic noise as the next and following main-scanning lines.
However, the conventional technique requires a memory for averaging and retaining pixel-by-pixel data to perform shading correction, which disadvantageously leads to an increase in circuit size.